(1) Field of the Invention
The present invention relates to an optical axis and focus adjustment mechanism for a semiconductor laser and a collimator lens for use in a laser scanning apparatus employed in a laser beam printer or the like.
(2)Description of the Related Art
A laser scanning apparatus provided with a semiconductor laser and a collimator lens is employed in a laser beam printer, a laser scanner or the like. Generally, a laser beam is radially emitted from a semiconductor laser and is converted into a parallel beam by a collimator lens for scanning. Since a focal length of the collimator lens is extremely shorter than that of an image-forming optical system which forms an image on an image carrier, a slight error in positioning the semiconductor laser leads to a considerable error on the image carrier.
In a laser beam printer and so on, such an error causes a recorded image to be blurred. Accordingly, it is required to position the semiconductor laser on the focus of the collimator lens (referred to as a focus adjustment hereinafter) and to position the semiconductor laser so that the optical axis of the laser beam coincides with the optical axis of the collimator lens (referred to as a optical axis adjustment hereinafter).
FIG. 1 is a plan view of a typical example of a conventional optical axis and focus adjustment mechanism. FIG. 2 is a cross section taken along the line II--II of FIG. 1 and FIG. 3 is an exploded perspective view. A mechanism 1 comprises a holding member 3 for holding a semiconductor laser 2 and a holding member 5 for holding a collimator lens 4. The holding member 3 has through holes 7a, 7b and 7c for receiving adjusting screws 6a, 6b and 6c loosely and has a screw hole 9 into which an adjusting screw 8 is screwed. On the other hand, the holding member 5 is provided with bosses 10a, 10b and 10c which have screw holes 11a, 11b and 11c, respectively. The adjusting screws 6a, 6b and 6c are inserted through the through holes 7a, 7b and 7c and are screwed into the screw holes 11a, 11b and 11c. The adjusting screw 8 is screwed into the screw hole 9 until its tip comes into contact with a surface of the holding member 5. The holding members 3 and 5 are energized by a spring 12 causing them to approach each other.
In the mechanism having the above construction, the optical axis adjustment is effected as follows. First, the adjusting screws 6a, 6b and 6c are loosened to move the holding member 3 on a plane vertical to the optical axis of the collimator lens 4. When the center of the semiconductor laser is positioned to be on the optical axis of the collimator lens, the adjusting screws 6a, 6b and 6c are tightened again to fix the holding member 3 to the holding member 5. Thus, the optical axis adjustment is completed.
In the focus adjustment, the adjusting screw 8 is screwed into the screw hole 9 to bend the holding member 3, whereby the distance between the semiconductor laser 2 and collimator lens 4 is adjusted in the direction of the optical axis.
The above mechanism, however, has the following problem. That is, the holding member 5 gets scratches on its surface at the time of the focus adjustment because the tip of the screw 8 is in contact with the surface. If the optical axis adjustment is effected after the focus adjustment, the tip of the adjusting screw 8 is caught in the above scratches, which hampers a smooth movement of the holding member 3 on the plane vertical to the optical axis.
In order to solve the above problem, it is conceivable to effect the optical axis adjustment prior to the focus adjustment. In this case, however, the focus adjustment affects the position of the semiconductor laser because the distance between the bending line of the holding member 3 and the semiconductor laser 2 is short. That is, according to the conventional technique, the focus adjustment and the optical axis adjustment are dependent on each other. Therefore, it has been impossible to effect these adjustments through a simple operation with high accuracy.